Layered Germanium–Selenium Compounds as Phonon–Glass Electron–Crystals: A Pathway to Enhance the Thermoelectric Performance

The early concept of a “phonon–glass electron–crystal” for enhancing the thermoelectric figure of merit (ZT) is explored theoretically in layered Ge–Se crystals, where phonon transport exhibits glass-like behavior. Ab initio lattice dynamics and the rigid electronic band method project an ultrahigh ZT = 4.04 at 1000 K along the a axis in the high-temperature GeSe₂ phase at an electron doping concentration of 10²⁰ cm^–3. Meanwhile, the low-temperature Ge₄Se₉ phase achieves a high ZT = 2.19 at 600 K along the a axis with an electron doping concentration of 6 × 10¹⁹ cm^–3. These maximal values reflect the ultralow lattice thermal conductivity, 0.168 W m^–1 K^–1 (GeSe₂, 1000 K) and 0.289 W m^–1 K^–1 (Ge₄Se₉, 600 K), and high power factor at optimized carrier concentrations along the a axis. Our calculations indicate a promising pathway for approaching the early concept of maximizing ZT, by tailoring carrier doping in layered crystals with glass-like phononic transport.